In the preparation of certain products such as, for example, porcelain used in dental restorative work, it is important that the moisture content of the product be properly controlled and maintained so as to avoid the problems of discoloration, deterioration and weakness. Heretofore, difficulty has been encountered in this regard because of the exposure to the atmosphere of the product for prolonged periods of time during the formation of the dental restorative and, thus, result in evaporation of the liquid from the product. Frequent and repeated manual replenishment of the evaporated liquid to the product was both awkward and inconvenient, involved guess-work on the amount of liquid to be replenished, and, thus, caused the consistency of the product to vary over a substantial range.